The use of lubricant fluids to reduce frictional losses in internal combustion engines is well known. Lubricant fluids typically contain either a hydrocarbon-based or synthetic principal lubricant oil, with additives selected to ensure that the composite lubricant fluid will serve to effectively lubricate relatively moving internal combustion engine parts under anticipated operating conditions.
Over time, through both analysis and experience, various characteristics of lubricant fluids have been better understood and improved. This is usually accomplished by adding one or more additives selected to adjust specific properties and monitoring the performance characteristics of the composite lubricant fluid. Additives such as viscosity index improvers are employed to control the viscosity, and pour point depressants are added as needed to control the freezing point of the composite lubricant fluid. Various detergent packages, corrosion inhibitors, and the like, may be added for their specific benefits.
A variety of multigrade lubricant fluids have been developed and are found to improve engine efficiency as measured by reductions in fuel consumption. In a study by McGeehan, J. A. "A Literature Review of the Effects of Piston and Ring Friction and Lubricating Oil Viscosity on Fuel Economy", SAE No. 780673, it is noted that multigrade lubricant fluids give slightly better fuel economy in reciprocating engines than do single-grade lubricant fluids. However, very little is known as to why improvements in fuel efficiency and reduced fuel consumption are achieved by the use of a multigrade lubricant fluid. Various explanations have been proposed to explain this disparity, but these, by necessity, until now, have been based on measurements of a single film thickness made in the main bearing of an internal combustion engine. See, for example, SAE Reports Nos. 869376, 880681, and 892151.
Other studies have considered the influence of cavitation in the lubricant fluid, in regions between relatively moving elements, as an important factor which determines the load bearing capability of the lubricant fluid film providing the lubrication. Theories concerning cavitation were first proposed by Reynolds in the early 1900s and these led to the development of the so-called Reynolds theory of lubrication. More recently, Coyne and Elrod, in "Conditions for the Rupture of a Lubricating Film: Parts I and II", Journal of Lubrication Technology, July 1970, have developed analyses which include the effects of surface tension in the lubricating mechanism. The influence of surface tension at the boundary conditions, and the task of specifying this in the analyses, thus adds a new parameter to both the analytical and experimental considerations.
The motor vehicle industry and the oil industry are both very concerned with energy conservation and oil consumption, and in the parameters involved in promoting engine efficiency and reducing oil consumption to avoid potential energy shortages. There is, therefore, significant interest in developing lubricant fluids and procedures for ensuring selected characteristics thereof for improved lubrication in internal combustion engines. To meet this need, it is necessary to develop an accurate understanding of the behavior of composite lubricant fluids, particularly where lubrication is provided to piston rings, both to develop a reliable model of the lubrication phenomenon and to enable the development of optimum lubricating fluid compositions. The goal of such efforts is to provide better lubricant fluids and an understanding of how to ensure that their desirable properties are maintained during prolonged use in internal combustion engines, to decrease friction-related losses, and to thereby increase engine efficiency and reduce fuel consumption.
The present invention is based on both analysis and empirical verification to provide improvements in lubricant fluid compositions and methods for ensuring efficient lubrication in internal combustion engines.
The following symbols and nomenclature are employed in the description of the invention.